In the manufacture of pipes which are to be threaded to each other, for the petroleum and natural gas industry, it is necessary to increase the wall thickness in the end region depending on the stress in the field of use and the type of thread cut. This is done in known fashion in the manner that a given length at the end of the pipe is hot upset in an upset machine and its wall thickness thereby increased. In this process, longitudinal upset seams are formed in the upset region on the outside of the end of the pipe since tie rods which press the two upset half shells to each other are elastically stretched during the upsetting and material flows into the gap produced thereby between the two upset half shells. As a result of this flow, and furthermore with an increasing number of upsets, the edges of the upset half shells also gradually become worn within a given region, so that the gap becomes larger and larger. The longitudinal upset seams disturb the subsequent working to precise dimensions on a lathe since the undisturbed flow of chips is prevented at this place and the impact-like loading leads to increased wear of the tool and the mounting of the tool holder is also impaired. Even if the loading of the machine tool is tolerated, these seams are nevertheless disturbing since they lead to a radial offset of the pipe end to be machined within the chuck of the lathe.
Up to now, as is known, the longitudinal upset seams which are formed have been eliminated by grinding. This manual cleaning of the hot-upset pipe ends is cost-intensive as a result of the expense for personnel, and the quality of the surface obtained is dependent also on the subjective judgment of the operator. For highly stressed pipe connections, manual grinding also constitutes a danger since, depending on the quality of the manual grinding work, injurious notches can be formed which may lead to failure of the thread connection.
An object of the present invention is to provide a method which can be integrated into the manufacturing process and by which it is possible, in a cost-favorable manner, to eliminate the longitudinal upset seams formed upon the hot upsetting of pipe ends so that, with the better surface, the subsequent precision machining can be increased in quality and output and the permanence of highly stressed pipe connections is not reduced.